The present invention relates to a system and a method for simultaneous, real time ultrasound imaging of biological tissue and measuring of blood flow velocity using the Doppler principle.
There exist in the marketplace devices for simultaneous ultrasound imaging of biological tissue and measuring of blood flow velocities based on the Doppler principle. The devices operate by time-sharing an ultrasound transducer between scan and Doppler modes of operation at a 1:1 ratio, that is, the ultrasound transducer is driven in each mode alternately. Operating the device at a 1:1 ratio enables a real time image of the biological tissue to be displayed, however, it restricts the upper velocity of blood flows that can be measured to relatively low values. Attempts to measure velocities greater than the upper limit produces the effect known as aliasing.
Operating the devices at a 1:N ratio where N&gt;1 enables the velocities of fast flowing blood flows to be measured but entails two disadvantages. First, the image of biological tissue is refreshed at too slow a rate for real time imaging. Second, the integrity of the Doppler power spectrum representative of the blood flow velocity is impaired. This impairment is due to the estimation of the Doppler signals which are missing when the device is operating in scan mode as now described with reference to the Doppler power spectrums shown in FIGS. 1a-1c.
The Doppler power spectrum shown in FIG. 1a displays a single harmonic at approximately 1 kHz as rendered by driving the ultrasound transducer at 8 kHz in the Doppler mode of operation only. In comparison, the Doppler power spectrum shown in FIG. 1b displays spurious harmonics at approximately -3 kHz, -1 kHz and 3 kHz in addition to the predominant 1 kHz harmonic as rendered by driving the ultrasound transducer at 8 kHz at a 1:4 ratio between the scan and Doppler modes of operation. The spurious harmonics can be quantified as contributing to a missing signal estimator (MSE) value of 4089.
To partly remedy this deterioration, the missing Doppler signals are typically estimated as a function of measured Doppler signals. Typical estimation techniques range from a simple linear interpolation between two immediately adjacent measured signals to more sophisticated estimation techniques as known in the art. As evidenced by the Doppler power spectrum of FIG. 1c, even a linear interpolation of missing Doppler signal manages to considerably lower the MSE value from 4098 to 318.3. However, it will be noted that the spurious harmonics in the Doppler power spectrum still remain albeit at a reduced energy.
Therefore, it would be highly desirable to have a system and a method for simultaneous, real time ultrasound imaging of biological tissue and measuring of blood flow velocities using the Doppler principle which improves the integrity of the Doppler power spectrums of the blood flow velocity measurements.
It would also be highly desirable that the system and the method be highly robust so as to be both equally applicable over a wide range of pulse repetition frequencies and computationally simple.